Fixed wing aircraft are typically fitted with deployable high lift devices for increasing the wing lift coefficient for take-off and landing. A slat is an example of a leading edge high lift device that can be mounted to the wing fixed leading edge and is deployed forwardly and downwardly with respect to the wing fixed leading edge as it moves from a retracted (stowed) position to one or more extended positions.
Each slat is typically mounted to the wing fixed leading edge by two or more spanwise spaced slat support tracks, which guide the movement of the slat relative to the wing fixed leading edge under the control of one or more slat actuators. Each slat support track is coupled to the slat by means of a lug, or knuckle, extending from the rearward face of the slat.
The slat has a generally concave rearward facing surface and the wing fixed leading edge has a generally convex forward facing surface such that the slat lies in close proximity to the wing fixed leading edge when stowed. The wing fixed leading edge therefore includes apertures to accommodate the lugs (and the support tracks) when the slat is retracted. The lugs will generally be clear of the apertures when the slat is fully extended (e.g. for landing) and may also be clear of the apertures when the slat is partially extended (e.g. for take-off).
Slats can be of either the “slotted” type, in which a slot is opened up between the slat and the wing fixed leading edge when the slat is at least partially extended; or the “sealed” type, in which the slat deployment follows closely the curvature of the wing fixed leading edge and the slat seals against the wing fixed leading edge when the slat is at least partially extended.
The aperture in the wing fixed leading edge, which is revealed when the slat knuckle moves out of the aperture, can cause undesirable flow characteristics around the wing leading edge region due to leakage flow through the aperture from the high pressure lower wing surface to the low pressure upper wing surface.
For good high-lift performance, it is desirable to close as much of the aperture as possible, regardless of whether the slat is of the slotted or sealed type. However, particularly in the case of the sealed slat, it is desirable to seal the aperture for at least the “take-off” configuration where the slat is partially extended.